This invention relates to piston-type fluid suspension systems of the type comprising a pressure cylinder adapted to receive a ported piston which is slidably disposed at one end of a reciprocable piston rod and divides the cylinder interior into two chambers, and more particularly to an improved hydraulic damper provided with an internal damping force sensor for adjusting damping characteristics of the damper both during compression and rebound strokes of the piston rod.
Whenever an automotive vehicle is in motion, springs in the vehicle suspension system compress and extend whereby to absorb road and maneuver inputs. However, unless damped or subjected to friction, the sprung and unsprung masses of the vehicle will continue to vibrate and such vibration will act to reduce the road-holding ability and riding comfort of the vehicle. To obtain better riding comfort and dampen unwanted vibrations, a hydraulic damper is connected between the body and the suspension of the vehicle. For the hydraulic damper to accommodate both compression and rebound of the piston rod, the hydraulic fluid must be allowed to pass between the two chambers, such as through the piston rod, or through a second cylinder encircling the first cylinder and defining a reserve chamber, and/or through the piston.
A ported piston limits the flow of damping fluid between the chambers when the piston rod is extended or compressed whereby to "smooth" or "dampen" vibrations transmitted from the suspension to the body. Generally, the greater the degree to which the flow of damping fluid is restricted by the piston, the greater the damping forces which are provided by the damper. A soft compression and rebound stroke is produced when the flow of damping fluid is relatively unrestricted. Conversely, a firm compression and rebound stroke is produced when there is an increased restriction in the flow of fluid in the damping chamber. Generally, the damping force is increased during extension for road-holding ability and decreased during compression for improvement in riding comfort.
Because different driving characteristics depend on the amount of damping forces the hydraulic damper provides, it would be desirable to have an arrangement in which the amount of damping force generated by the damper is adjustable and remotely controllable. In U.S. Pat. No. 4,313,529, issued Feb. 2, 1982 to Kato et al., an electromagnetically operated valve allows fluid to pass through the piston rod and between the chambers. In U.S. Patent Application Ser. No. 322,774, filed Mar. 19, 1989, an electromagnetically controlled solenoid opens an additional fluid path in the piston assembly. These references show that both a monotube (e.g., Kato et al.), or concentric multi-tube cylinders (e.g., the Patent Application) are suitable. The disclosure of each noted reference is specifically incorporated herein by reference.
An object of this invention is provision of means to detect instantaneous damping forces across a piston.
Another object of this invention is provision of a suspension system of the type described wherein apparatus is provided for controlling the amount of damping fluid flowing between the upper and lower portions of the working chamber to adjust the damping characteristic of the suspension system.
Yet another object of the present invention is provision of apparatus for controlling a hydraulic damper in which displacement of the piston assembly relative to the piston rod determines the occurrence of a compression or rebound stroke and allows a control means to adjust the pressure differential between the upper and lower portions of the working chamber (i.e., the pressure difference across the piston).
To achieve the above and other objects, there is provided a hydraulic damper of the type including a cylinder having an interior cavity to store damping fluid, a piston rod guide adjacent to one end of the cavity, a piston rod sealably and reciprocably inserted into one end of the cylinder and extending through the guide unit, and a piston unit connected to the piston rod and dividing the cavity into a compression chamber and a rebound chamber. A damping force generating mechanism is provided in the form of passageways which extend through the piston unit and/or through the piston rod and which operate to restrictively pass fluid between the compression and rebound chambers.
In accordance with this invention, the piston rod includes a generally hollow portion, an electrical coil is wound coaxially around the piston rod, the piston unit is slidably disposed about the piston rod for reciprocable axial movement between a pair of axially spaced shoulders, and a pair of precompressed helical coil springs position the piston unit between the shoulders and resist movement of the piston unit relative to the piston rod. The piston unit is provided with a pair of axial passages and associated valve plates which restrictively allow (or inhibit) fluid to flow between the chambers, and a cylindrical "target" or "spoiler" sleeve extending in encircling relation about the piston rod. An electrically controllable valve is operably connected to the inner end of the piston rod for regulating flow of fluid through the piston rod and between the chambers in response to an electrical control signal.
An oscillator external to the damper imposes an alternating electrical voltage on the electrical coil, resulting in an alternating electrical current to energize the coil which in turn generates a magnetic field about the spoiler sleeve. A current sensor and electronic control module, remote to the piston assembly, detects relative movement of the sleeve relative to the magnetic field, which is reflected in a change in the electrical current. This change in current is transmitted to an electronic control module, remote to the damper, which in turn generates a control signal, in response to the relative movement. The electrically controllable valve responds to this control signal to allow a regulated amount of hydraulic fluid to flow through the piston rod and between the chambers.
Detecting instantaneous relative movement between the piston assembly and piston rod can advantageously be converted into a signal which is indicative of the change in static pressure acting on the piston assembly, which signal can be converted into a control signal used to change flow between the chambers and effect rebound or compression of the vehicle.
Further, it is to be appreciated that the signal detected by the sensor, indicating the relative velocity between piston rod and piston assembly, can be directly converted into acceleration by electronic apparatus readily available in the control art.
Advantageously, acceleration determination will indicate the upper and lower chamber pressures and the damping force generated across the piston assembly. As such, the rebound or compression stroke of the piston rod can be controlled.
Advantageously, the immersion of the piston unit and sensor in the working fluid ensures adequate damping of its motion relative to the piston rod, so as to prevent acoustical ringing.
Other objects, advantages and features of the present invention will become apparent to one skilled in the art upon reading the following description and dependent claims, taken in conjunction with the following drawings.